Power line communication (hereinafter, referred to as “PLC”) that uses existing power lines as transmission paths, for performing communication has a problem of a leakage electromagnetic field radiated from the power lines during communication. A PLC signal is inputted in a differential mode to the power lines. However, there is a case where a common mode signal is generated by circuit unbalance in the power lines.
Common mode signals are transferred through two power lines in a common mode, and form current loops with the ground being feedback paths. It is possible that a leakage electromagnetic field radiated from a power line is proportional to the magnitude of a common mode current. Therefore, in order to reduce the leakage electromagnetic field, the common mode current flowing through the power line needs to be suppressed.
FIG. 4 is a diagram showing a circuit configuration of a general conventional modem apparatus. In FIG. 4, the conventional modem apparatus is provided with an amplifier 1, a signal transformer 3, and a common mode choke 2A. In the modem apparatus which outputs a PLC signal to power lines 4, a method of using the signal transformer 3 and the common mode choke 2A is generally adopted at an output end of the modem apparatus in order to suppress the common mode current. When the signal transformer 3 operates ideally, an unbalanced signal at a primary side is transformed into a balanced signal, and a common mode component does not flow.
FIG. 5 is a diagram showing a circuit configuration of another conventional modem apparatus (for example, see Patent Document 1). In FIG. 5, another conventional modem apparatus 10A is provided with a transmission circuit 1A that includes an amplifier, a common mode detecting circuit (balancing detector) 5A, and two balance circuits (balancing adjusting circuits) 2B. The configuration of FIG. 5 shows an example of suppressing the common mode current. Specifically, the balance circuits (balancing adjusting circuits) 2B are provided for power lines 4 serving as transmission paths, and the common mode detecting circuit (balancing detector) 5A controls the balance circuits 2B such that the levels of common mode signals become smaller.    Patent Document 1: JP 2004-80441 A